An amplifier is required in wireless communication to amplify a transmission signal. The amplifier generates nonlinear distortion as a nonlinear output level and an output phase with respect to an input level. Input vs. output characteristics showing nonlinear distortion have AM-AM characteristics showing the output level with respect to the input level, and AM-PM characteristics showing the output phase with respect to the input level. However, the amplifier is daringly used in an area for generating the nonlinear distortion to improve electric power efficiency. The nonlinear distortion generally causes unnecessary electric power emission over a band three to five times (seven times in a bad case) as much as a band width of the transmission signal. FIG. 11 is a spectrum showing one example of the transmission signal including distortion. The axis of abscissa shows a relative frequency with respect to a carrier frequency, and the axis of ordinate shows a relative level with respect to a desirable wave. In addition to the desirable wave, −IM7 (a seventh order higher harmonic wave of a negative side), −IM5 (a fifth order higher harmonic wave of the negative side), −IM3 (a fifth order higher harmonic wave band of the negative side), +IM3 (a third order higher harmonic wave of a positive side), +IM5 (a fifth order higher harmonic wave of the positive side) and +IM7 (a seventh order higher harmonic wave of the positive side) are shown.
Accordingly, a distortion compensation amplifying device for compensating the nonlinear distortion is required. As a system of signal processing for compensating the nonlinear distortion, there are an FF (Feed Forward) system and an APD (Adaptive Pre-Distortion) system. Each system is embodied at a commercial use level and a trial manufacturing level, and is effective. In the FF system, only distortion of an amplifier output is extracted and is subtracted from the amplifier output. In the APD system, distortion characteristics are given in advance before an amplifier input, and are cancelled with respect to the amplifier. In the APD system, all can be realized in digital signal processing. Therefore, the APD system is promising in compact formation and low cost formation. Further, in the APD system, an adaptive algorithm for following a change in characteristics of the amplifier such as temperature characteristics and a change with the passage of time, etc. is used.
FIG. 12 is a block diagram showing one example of the construction of a conventional distortion compensation amplifying device. This distortion compensation amplifying device has an amplifying section 1, a distributing section 2, a distortion detector 103, a compensation coefficient arithmetic section 104, an envelope line detecting section 11, a reverse characteristics memory section 12, a delay section 13 and a PD (pre-distorting) section 14.
There are several kinds of adaptive algorithms. For example, there is a method for changing a distortion compensation coefficient while a distortion signal is monitored (e.g., see patent literature 1). The reverse characteristics memory section 12 stores the AM-AM characteristics and reverse characteristics of the AM-PM characteristics provided in the amplifying section 1 in advance. The envelope line detecting section 11 outputs an envelope of the transmission signal as an address of the reverse characteristics memory section 12. The reverse characteristics memory section 12 outputs the reverse characteristics stored to this address to the PD section 14. The PD section 14 gives the reverse characteristics to the transmission signal delayed in the delay section 13. The amplifying section 1 amplifies an output of the PD section 14. The distributing section 2 distributes an output of the amplifying section 2, and outputs one of this output to the exterior and outputs another of this output to the distortion detector 103. The compensation coefficient arithmetic section 104 updates contents of the reverse characteristics memory section 12 by using an adaptive algorithm so as to minimize distortion detected by the distortion detector 103.
For example, patent literature 2 shown below is known as a relative prior art of the present invention. This pulse wave detector can measure an accurate pulse number even when an arithmetic means low in calculation ability is used.
Patent literature 1: JP-A-2004-128921
Patent literature 2: JP-A-2004-121625 (pp. 4-7 and FIG. 1)
Patent literature 3: JP-A-9-138251